The present invention generally relates to photolithography. More particularly, it involves a photolithographic process, and apparatus and material for use therein, for producing etched or eroded areas or holes in a selected pattern on or in the surface of fine workpieces, such as small diameter tubes.
Many applications, particularly in the biomedical field, require a flexible, three dimensional member. Stents, for example, are commonly used in angioplasty procedures for the treatment of coronary artery disease. The afflicted artery is dilated/expanded in these procedures through the use of an angioplasty balloon. Without artificial reinforcement, however, the balloon-expanded vessel has a tendency to constrict to its previous obstructed internal diameter shortly after the angioplasty procedure. By implanting a stent along the expanded portion of the vessel, the vessel is provided with sufficient radial reinforcement to prevent the vessel from constricting.
A variety of conventional techniques exist for manufacturing stents. One such technique involves forming wire members about a cylindrical support member, such as a mandrel, so as to overlap or intertwine with one another. The wire members are joined together at the intersection points using conventional bonding techniques, such as welding, blazing or soldering. Once the wire members are suitably interconnected, the mandrel is removed from the wire members, thereby leaving an elongated wire mesh tube having a plurality of openings defined between the individual wire members. Another conventional manufacturing technique is to cut a pattern of openings into a thin-walled stainless steel tube using electromechanical or laser machining techniques.
One problem with these conventional techniques, however, is that they are expensive and time consuming. This problem is compounded because very little cost savings can be achieved by making a large quantity of stents. In addition, stents made by conventional techniques, particularly the techniques using mechanical and electromechanical processes, have practical size limitations due to the number of holes or bonds (frequently  greater than 4000) required to make the leading 5-6 inches of the stent sufficiently flexible.
One possible solution to these problems is photolithography. Conventional photolithographic techniques are desirable because a manufacturer can use a single mask in conjunction with well-known etching processes to inexpensively produce a large number of stents. Photolithography is also desirable because marginal production costs are substantially independent of the complexity of the desired pattern. That is, although a large number of openings will increase the cost of making the mask, the number of openings will not greatly affect the costs directly associated with making an additional stent.
One problem with conventional xe2x80x9cstate of the artxe2x80x9d photolithographic techniques, however, is that they use a flat mask. This flat mask cannot project a useful image onto the three dimensional and/or nonplanar structures used in medical devices without making complex and expensive changes to the mask pattern. Another problem with conventional xe2x80x9cstate of the artxe2x80x9d photolithography is that the mask patterns are generally fabricated by sputtering metal or other opaque materials onto a smooth glass plate. These plates, however, are fragile and are susceptible to scratching.
Clearly, there is a need for a simple, inexpensive, and robust method and apparatus that can produce a usable photoresist pattern for three-dimensional process requirements. In particular, there is a need to produce usable patterns of photoresist on the surface of relatively fine cylinders, (i.e., 10-15 mil OD tubes or wires) with the objective of permitting the etching by chemical or plasma techniques of a desired texture (wires) or holes (tubes).
The present invention provides a photolithographic process for producing etched patterns on the surface of fine tubes, wires, or other three dimensional structures. In particular, the present invention produces usable three-dimensional photoresist patterns on the surface of fine cylinders. This photoresist pattern can permit chemical or plasma etching of a desired texture on wires or of desired holes in tubes. One embodiment of the present invention comprises the following apparatus and material: a cleaning agent, a photoresist agent or solution, a mask, a developer solution and a resist remover. An appropriate exposing light source is provided. Each of the preceding may be selected or matched with the composition and configuration of the workpiece and with the solutions or chemicals. The selected mask and/or mask holder may be machined to carry or exhibit a selected pattern of slots or exposure openings, and may include one or more alignment notches, pins or the like. The present invention is well-suited for use on fine cylindrical or tubular workpieces, in which case a curved mask is used. For tubular workpieces, the interior or lumen thereof may be protected from being plugged or occluded by removably lodging a plug therein or by filling it with a suitable removable material.
The present invention also provides a photolithographic method of providing a workpiece with an etched area comprising applying a suitable photoresist material to the workpiece, providing a mask with a desired pattern corresponding to the etched area, aligning the mask and the workpiece, and exposing and developing the photoresist material. More particularly, the process or method of the present invention comprises providing a subject workpiece, applying a suitable photoresist solution to the workpiece, providing a mask with a desired pattern, aligning the mask and the workpiece, exposing the resist, treating the exposed resist in a developer solution, etching the workpiece in an appropriate manner, and removing the resist. Appropriate intervening and complimentary steps may be included in the process of the present invention. For example, the workpiece may be cleaned and dried prior to the application of the photoresist solution, the resist coated workpiece may be baked or otherwise cured, and the workpiece may be rinsed and/or dried after immersion in the developing solution and/or after the application of the resist remover. These steps may involve chemical solutions or agents tailored to the workpiece material, and/or to the selected photoresist, developing and resist removal solutions. The conditions of and in which the process takes place may be optimized for specific applications or workpieces.
Some embodiments of the present invention include a photolithographic process for producing a selected pattern on a nonplanar surface of a workpiece using at least one mask to define the selected pattern. The process comprises the acts of applying a photoresist material to the workpiece and of aligning the mask with a nonplanar surface of the workpiece. The mask in these embodiments may correspond to the nonplanar surface.
The present invention also includes a photolithography apparatus for forming an offset pattern in a tubular workpiece and a photolithographic method of forming a radially spaced offset pattern on a three dimensional workpiece. The photolithography apparatus in some embodiments comprises a stage having a hemicylindrical substrate channel adapted to receive a tubular workpiece, a hemicylindrical mask defining at least one aperture, a first stop associated with the stage and adapted to position the workpiece in a first position relative to the mask, a second stop associated with the stage and adapted to position the workpiece in a second position relative to the mask, and an exposing light source adapted to illuminate the tubular workpiece through the at least one aperture. The photolithographic method in some embodiments comprises the acts of placing a workpiece on a stage, jogging the workpiece against a first stop, exposing a first portion of the photo-sensitive coating through a mask, rotating the workpiece, jogging the workpiece against a second stop, and exposing a second portion of the photo-sensitive coating though the mask. The workpiece in this method has a photo-sensitive coating.
An advantage of the present invention is that it may be used to produce a selected pattern of holes on or in a workpiece having a small cross-sectional area or diameter more cost efficiently than machining. Another advantage of the process of the present invention is that it is well suited for producing a selected pattern of a large number of offset holes on or in a fine cylindrical or tubular workpiece. For example, it may be used to render a workpiece or portion thereof more flexible by providing for the partial or complete erosion of the surface of the workpiece in a pattern comprising a plurality of opposed, offset eroded areas about the circumference of the workpiece for a selected length thereof.
These and other features and advantages of the present invention will become more fully apparent and understood with reference to the enclosed descriptive material and drawings.